We’ll assume that you don’t need guidance about the obvious: don’t drop it in a lake, or from a helicopter, don’t pour it full of soda or beer, and so on.
A few more pointed do not’s:
Never, never, never run the amp with no speaker plugged in. This can cause major damage.
Do not flip the power switch off, then back on rapidly. This can cause power supply damage.
Never replace a burned out fuse with a bigger-amperage one. Remember – there was a reason the first one burned out, usually protecting something more expensive. Putting a bigger fuse in will just rachet up the power level until something really vital burns out. If the second equal-rating fuse pops, turn it off and get a tech to look at it.
Never ignore signs of high heat inside – a wisp of smoke or a burning smell is NOT normal.
Your amp produces lots of heat, and will continue to do so even if you block the fresh air vents. Blocking the vents will just allow the amp to heat to the point that you get to buy some very expensive repairs.
Never ignore a red glow other than the small orange ends of the filaments. A red glow over a large part of the internal plates of the output tubes means they’re about to melt (yes, really melt – heat is our enemy). If you notice this, shut it down and get a tech to help you find out what it wrong.
Correspondingly, you can do the following without too much worry:
Add another speaker into the “external speaker” jack; a mismatched speaker load won’t kill it, while an open circuit (disconnected speakers) may do so.
Overdrive the stuffings out of it. Tubes are very forgiving of massive overdrives, unlike solid state stuff. As long as they tubes don’t overheat or stay overdriven for long periods, it’s not fatal.